English swimmers, including Fran Halsall and Rebecca Adlington, are among those who have been struggling with the virus, according to reports.

Games Federation president Mike Fennell said officials will test both the main pool and the warm-up pool at the Dr S.P. Makherjee Aquatic Complex in Delhi.

Asked if the swimming, which began its fourth of six days, might be cancelled or moved if tests showed the pools were unsafe he said: "I would not like to speculate about this immediately. If there is something unsafe, you cannot swim in that water. It is a matter we have to deal with a great deal of urgency."

There was no immediate indication of any testing being undertaken at the aquatic centre. The synchronised swimming event was underway in the dive pool, while the main pool was idle ahead of the early evening's finals session.

During the meet, photos taken by an underwater camera in the main pool were mostly murky.

An Australian swim team spokesman said "about a half-dozen" swimmers had been affected by stomach problems. "Our doctors are looking into it ," he said.

England team spokesman Caroline Searle said between seven and 10% of England's 541-strong delegation had been affected by a "mild 24-hour stomach condition."

The games bring together more than 6,000 athletes and officials from 71 countries and territories. But construction delays, corruption allegations, concerns about security and heavy monsoons put preparations for the games way behind schedule, with complaints about unfinished and filthy accommodations in the athletes' village embarrassing the hosts.